The ability of Peyer's patch cells to present antigens of varying chemical composition is under investigation. The adherent cells from Peyer's patches (APPC) have been isolated and found to contain Ia antigens and do not present soluble antigen to sensitized T cells. The relationship between the ability of APPC to present antigens and the capacity of these antigens to induce systemic tolerance when given orally is under study. With some antigens, such as soluble proteins, systemic tolerance can be induced and antigen-specific splenic suppressor cells are found on adoptive transfer. Systemic tolerance is accompanied by an active secretory response to the specifc antigen. The mechanisms and genetics of oral tolerance to human gamma globulin (HGG) is being studied in different strains of mice. The ability of HGG to induce suppressor cells for gamma G and gamma M in Peyer's patch cells is currently under investigation.